Florida is the purported site of the fountain of youth. But Miami Dolphins Coach Tony Sparano swears the waters must be closer to San Francisco.

Because the more Sparano prepares to face Isaac Bruce, 36, the more he wonders when the 49ers receiver is going to act his age.

“It’s amazing watching him, it really is,” Sparano said Friday. “You hear, ‘Oh, he lost a step.’ I don’t see it. I see him as quick as he’s ever been.”

Heading into Sunday’s game at Dolphin Stadium, Bruce leads the 49ers with 45 catches, 695 yards and six receiving touchdowns.

Bruce, who was born in Fort Lauderdale, Fla., needs 131 yards to surpass Tim Brown (14,934) for second on the NFL’s all-time receiving list. If he doesn’t surpass Brown on Sunday, he might do it the following week in St. Louis — against the team he played for in his first 14 seasons.

With running back Frank Gore (sprained ankle) expected to be out, the 49ers might lean more heavily on their passing game.

The Dolphins rank 11th against the run but only 21st against the pass. That’s why Sparano is concerned about Bruce.

“He’s a tremendous technical route runner,” he said. “He knows how to set you up before he attacks you. He has a plan every time he approaches a defender that way. There’s not going to be a coverage that you’re going to throw at him that he doesn’t understand.

“You’ve got a full-time challenge on your hand when you’re playing against this guy. He knows all of the tricks of the trade in there, and that’s all you need sometimes to gain a little bit of space.”

49ers Coach Mike Singletary continued to list Gore as questionable on the injury report. Gore did not practice all week and spent Monday and Tuesday on crutches. Barring a miracle comeback, DeShaun Foster will start against the Dolphins with Michael Robinson serving as the backup running back.

Allen Rossum (ankle), who last played Nov. 23 against Dallas, is listed as probable and expected to return punts Sunday. If he can’t go, receiver Dominique Zeigler is a candidate to return punts. “He’s a guy that we trust,” Singletary said. “He’s been very reliable. So I think he would do just fine.”

Gore, who grew up in the Miami area, isn’t the only 49er with ties to South Florida. Tony Nathan, the 49ers running backs coach, played nine seasons for the Dolphins and ranks fifth on the team’s all-time rushing list.

Nathan led the Dolphins with 10 receptions in Super Bowl XIX against the 49ers. “That was a tough sled,” Nathan recalled Friday. “We thought we could do it. We earned the right to be there. Unfortunately, we couldn’t put the icing on the cake.”

Dolphins General Manager Jeff Ireland was a ballboy in the Chicago Bears training camps when Singletary played there. … The 49ers have not played at Miami since Nov. 20, 1995, when they won 44-20. … The 49ers have 55 plays of 20 yards or longer. The only teams with more are the Houston Texans (60) and the New Orleans Saints (58).

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